1. Field of the Invention
The present invention relates to membrane separation of helium from natural gas.
2. Related Art
The only source of Helium is from natural gas. Helium is typically present in natural gas at below 0.5 mol % levels and is mostly extracted as crude Helium across liquid natural gas (LNG) trains. This crude Helium, containing about 20-30 mol % Helium, is then enriched either by cryogenic distillation or via a PSA to make 99.9999 mol % Helium.
Small gas molecules such as Helium are well known to be more permeable through glassy polymer membranes than methane or N2. Hence, membranes can be considered for Helium recovery from natural gas. However, Helium is typically found in very low concentrations and it is difficult for a single stage membrane to achieve commercially viable levels of recovery and/or selectivity.
In general, recovery of dilute components by membranes requires multiple stages in order to achieve high purity. Other mass transfer operations, such as distillation can produce high purities by means of multiple stages. Unfortunately, membrane processes are expensive to stage since each additional stage involves permeate recompression with the attendant operating and capital costs of the compressor.
Methods of optimally staging membrane processes have been extensively studied in the academic literature. Examples of this work include Agarwal, et al., (“Gas separation membrane cascades II. Two-compressor cascades”, Journal of Membrane Science 112 (1996) 129-146) and Hao 2008 (“Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams”, Journal of Membrane Science 320 (2008) 108-122).
Staged membrane operations are also practiced commercially. An example is the well-known 2-stage process described by WO 12050816 A2. In this scheme, permeate from a first membrane stage (or from a section of a first membrane stage) of is re-compressed and processed by a second membrane stage. The second stage permeate is achieved at higher fast gas purity. The second stage residue is recycled to the first stage membrane feed.
Permeate refluxing is described in some versions of membrane column work by Tsuru, et al. (“Permeators and continuous membrane columns with retentate recycle”, Journal of Membrane Science 98 (1995) 57-67). In this context, permeate refluxing is practiced on a single membrane stage with refluxing of a fraction of the permeate, then re-compressing that fraction and recycling it to either the feed gas or as a sweep gas. This permeate refluxing scheme is not appropriate for handling a high volume gas as the membrane area required for combined high purity and high recovery is very high.
It is an object of the invention to provide a method for separation of helium from natural gas using membranes that achieves a satisfactorily high helium recovery while attaining a minimum heating value in the purified natural gas without requiring multiple compressors.